Re: First draft of the European Template for IP number requests
- Date: Thu, 10 Dec 92 14:31:44 N
> > Looking at all possible subnet masks, you get:
> >
> > subnet mask | # subnets | # hosts/subnet | total # hosts | usage
> > -------------+------------+----------------+---------------+------
> > 1:7 | not allowed| | |
> > 2:6 | 2 | 62 | 124 | 49 %
> > 3:5 | 6 | 30 | 180 | 71 %
> > 4:4 | 14 | 14 | 196 | 77 %
> > 5:3 | 30 | 6 | 180 | 71 %
> > 6:2 | 62 | 2 | 124 | 49 %
> > 7:1 | not allowed| | |
>
> Just one probably silly question - why should the subnet mask 1:7 not
> be allowed? If I get it right, you speak of netmask like 255.255.255.128.
Your interpretation is correct. The mask 1:7 (as 7:1) is not allowed
according to the recommendation to use all zeroes and all ones *neither* in
the host part, *nor* in the subnet part.
From RFC1009 [Requirements for Internet gateways], page 6:
RFC1009> The bit positions containing this extended network number are
RFC1009> indicated by a 32-bit mask called the "subnet mask" [21]; it is
RFC1009> recommended but not required that the <Subnet-number> bits be
RFC1009> contiguous and fall between the <Network-number> and the
RFC1009> <Host-number> fields. No subnet should be assigned the value
RFC1009> zero or -1 (all one bits).
> Which should let you have two subnets of 126 hosts each.
Similar reasons might have lead to the following
B) In order to prevent implementation problems, network numbers ending
with 0 or 255 should NOT be reassigned.
found in ripe-72.txt . Just walking off the topic ...
Peter
